In the prior art, aluminum is well recognized for its corrosion resistance. AA1000 series aluminum alloys are often selected where corrosion resistance is needed.
In applications were higher strengths may be needed, AA1000 series alloys have been replaced with more highly alloyed materials such as the AA3000 series type aluminum alloys. AA3102 and AA3003 are examples of higher strength aluminum alloys having good corrosion resistance.
Aluminum alloys of the AA3000 series type have found extensive use in the automotive industry due to their combination of high strength, light weight, corrosion resistance and extrudability. These alloys are often made into tubing for use in heat exchanger or air conditioning condenser applications.
One of the problems that AA3000 series alloys have when subjected to some corrosive environments is pitting or blistering corrosion. These types of corrosion often occur in the types of environments found in heat exchanger or air conditioning condenser applications and can result in failure of an automotive component where the corrosion compromises the integrity of the aluminum alloy tubing.
In a search for aluminum alloys having improved corrosion resistance, more highly alloyed materials have been developed such as those disclosed in U.S. Pat. Nos. 4,649,087 and 4,828,794. These more highly alloyed materials while providing improved corrosion performance are not conducive to extrusion due to the need for extremely high extrusion forces.
U.S. Pat. No. 5,286,316 discloses an aluminum alloy with both high extrudability and high corrosion resistance. This alloy consists essentially of about 0.1-0.5% by weight of manganese, about 0.05-0.12% by weight of silicon, about 0.10-0.20% by weight of titanium, about 0.15-0.25% by weight of iron, with the balance aluminum and incidental impurities. The alloy preferably is essentially copper free, with copper being limited to not more than 0.01%. This alloy is essentially copper free with the level of copper not exceeding 0.03% by weight.
Although the alloy disclosed in U.S. Pat. No. 5,286,316 offers improved corrosion resistance over AA3102, even more corrosion resistance is desirable. In corrosion testing using salt water--acetic acid sprays as set forth in ASTM Standard G85 (hereinafter SWAAT testing), condenser tubes made of AA3102 material lasted only eight days in a SWAAT test environment before failing. In similar experiments using the alloy taught in U.S. Pat. No. 5,286,316, longer durations than AA3102 were achieved. However, the improved alloy of U.S. Pat. No. 5,286,316 still failed in SWAAT testing in less than 20 days.
An improved aluminum alloy has been developed which overcomes the drawbacks noted above in prior art corrosion resistant alloys. This improved alloy is an AA3000 series type alloy having controlled amounts of copper, zinc and titanium. The improved alloy is especially suited for applications requiring both hot formability and corrosion resistance. The alloy consists essentially of, in weight percent, an amount of copper up to 0.03%, between about 0.05 and 0.12% silicon, between about 0.1 and about 0.5% manganese, between about 0.03 and about 0.30% titanium, less than 0.01% magnesium, less than 0.01% nickel, between about 0.06 and about 1.0% zinc, an amount of iron up to about 0.50%, up to 0.50% chromium, with the balance aluminum and inevitable impurities. Further, an example of the alloy is described in which the copper is about 0.008% or less; the titanium is between about 0.07 and 0.20%; the zinc is between about 0.10 and 0.20%; and iron is between about 0.05 and 0.30%. This improved alloy is disclosed in U.S. patent application Ser. No. 08/659,787 filed on Jun. 6, 1996, which is hereby incorporated in its entirety by reference.
While the improved alloy offers superb corrosion resistance and hot formability, particularly when extruded into tubing, the improved alloy does not always provide adequate performance when subjected to further cold deforming and optional annealing. Often times, the improved alloy is cold drawn after hot deforming or cold drawn and annealed. The cold drawn alloy is susceptible to necking or local deformation which can cause product breakage and an unacceptable surface finish, e.g. stretcher strains or orange peel. One of the causes of the necking is insufficient resistance to deformation or softness once the material passes the yield point but has not reached the ultimate tensile strength. In the metallurgical arts, the ability to resist local deformation can be measured by the "n value". The n value generally measures the difference between the yield point and the ultimate tensile strength. Since this value is well recognized in the art, a further description is not deemed necessary for understanding of the invention
In view of the drawbacks of the improved alloy discussed above, a need has developed to provide a new and improved alloy which has not only good corrosion resistance and hot formability but also bendability and drawability. In response to this need, the present invention provides an aluminum alloy material which has controlled amounts of manganese, magnesium and zirconium and is suitable for not only corrosion resistant applications of hot deformed materials but also materials that are hot deformed and cold worked, with or without annealing and subsequent cold deforming.